Wet-process manufacturing of phosphoric acid is so standardized that its procedures are well known and are described as conventional in authoritative texts. An accepted reputable authority upon this subject is Pierre Becker, PHOSPHATES AND PHOSPHORIC ACID--Raw Materials, Technology, and Economics of the Wet Process, published by Marcel Dekker, Inc. (New York, 1983). Of particular interest is chapter 9 thereof, entitled "What to Do with Gypsum" (pp. 471-496). The present inventors consider Becker's identified treatise as the prime authority on conventional wet-process phosphoric acid practice, from which the present invention departs as described below.
Such conventional processing produces by-product gypsum stacks from which very acidic water drains into extensive cooling ponds, often measuring several hundred acres each. Conventional gyp[sum] pond water is extremely acid, usually having a pH of about 1.8-2.0. Also, it is composed of weak phosphoric acid, as in a range from 1.3-2.5% P.sub.2 O.sub.5, fluosilicic acid and soluble fluorides ranging from 0.5-1.8%, sulfuric acid and soluble sulfates ranging from 0.5-1.6%, plus soluble contaminating metal salts and radioactive compositions Conventional acid gypsum pond water retains in solution--and so is likely to pollute ground water with--residual phosphoric acid and soluble fluorides, dissolved metal impurities and radioactive compounds, and it emits fluoride-containing gases. This is a serious drawback of conventional wet-process phosphoric acid manufacturing.
Attempts to limit resulting air, water, and soil contamination have had limited success. Randolph in U.S. Pat. No. 3,625,648 proposed a rather diffuse range of neutralization of acidic gypsum pond waters effective to reduce some contaminants by precipitation. The present inventors also undertook to reduce fluoride contamination, Palm as in U.S. Pat. No. 3,699,212 and Hartig as in U.S. Pat. No. 3,720,757, and jointly they subsequently developed a two-stage neutralization of recirculated acid waters, as in U.S. Pat. No. 4,320,012.
Other examples of ameliorative efforts include treating gypsum pond waters to remove metallic ions as in Mills U.S. Pat. No. 4,303,532; to recover calcium fluoride as in Hirko & Mills U.S. Pat. No. 4,171,342 or O'Neill U.S. Pat. No. 4,374,810; or to remove calcium fluoride so as to provide a reduced fluoride solution for use in wet ball-milling of phosphate rock as in O'Neill et al. in U.S. Pat. No. 4,472,368. Hirko et al. U.S. Pat. No. 4,171,342; Zibrida in U.S. Pat. Nos. 4,657,680 and 4,698,163; and Murray et al. in U.S. Pat. No. 5,112,499.
The industry and the inventors, despite such efforts, have not successfully eliminated the dependency of conventional wet-process phosphoric acid manufacturing upon acid pond water with its load of contaminants threatening pollution of adjacent air, soil, and water.
The present invention, while retaining benefits of the present inventors' identified early joint invention, is directed to modifying the wet-process manufacturing of phosphoric acid with the result of substantially eliminating pollution attributable to conventional gypsum pond waters and minimizing the loss of P.sub.2 O.sub.5 while doing so.